Optical power splitter is a component widely-used in optical communication networks to provide a variety of applications such as optical signal routing and optical power allocation. With advance of agile optical networks, a variable optical power splitter (VOPS) becomes imperative to improve network scalability and flexibility. The VOPS can be regarded as the combination of an optical switch and a variable optical attenuator, and thus possesses the functions of both components. In long-haul and metropolitan networks, the VOPS can be employed as an optical switch to provide optical line protection of the networks or an optical coupler to provide optical signal performance monitoring. While in access networks such as passive optical networks, the VOPS can be deployed between optical line terminal and optical network units (ONUs) to distribute optical powers to each ONU dynamically by changing the power splitting ratio. Similarly, the VOPS can also be incorporated into an ONU to allocate optical powers among its connected end users according to their actual distance from the ONU.
A few schemes have been proposed to develop the VOPS, based on either waveguide or crystal technologies. The waveguide technologies employ a directional coupler as the key component. The variable power splitting ratio between the output ports of the coupler is achieved by changing the coupling length or separation of waveguides. However, the coupling length is dependent on the signal wavelength and therefore it is not suitable for broadband application. On the other hand, the separation of the waveguides requires precise mechanics and thus makes the VOPS rather costly.
The crystal-based VOPS can be based on solid-state crystal or liquid crystal. The solid-state crystal-based VOPS makes use of the electro-optic and/or acousto-optic effects of the crystals to achieve light modulation and power splitting. Similar to the aforementioned directional coupler, the resonance conditions in crystals rely on the signal wavelength therefore the VOPS can only be operated in a narrow wavelength range.
U.S. Pat. No. 5,740,288 discloses a variable polarization beam splitter made of a liquid crystal cell and a birefringence crystal. The liquid crystal cell functions as a polarization controller through which the signal power is allocated dynamically between two orthogonal states of polarization. The two orthogonal states of polarization are then split spatially by the birefringence crystal. In U.S. Pat. No. 7,113,279, a polarization diversity scheme is incorporated into a liquid-crystal-based VOPS. The power splitting ratio of the VOPS is no longer dependent on the state of the polarization of the input signal, at the expense of extra polarizing beam splitter and combiner. Moreover, the performance of liquid crystal is temperature dependent therefore the VOPS will suffer from the variation of ambient temperature. Additional temperature controller or stabilizer may be used. Nevertheless, they will increase the cost accordingly. Accordingly, low cost, temperature independent, broadband VOPSs are desired.
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